use static constexpr and braced initialization in policy.h
Summary:
This is a backport of core#25388
Most of the PR is related to chained transaction limits, so not applicable to us.
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D15038